Cannot find the faces associated with this item.

When I attempt to apply a Fluid Subdomain to an internal fluid volume (on faces on different parts in my assembly), I get this Rebuild Error: "Cannot find the faces associated with this item." Can anyone offer any help? I don't know what the problem is... I have done the check geometry feature within flow simulation and it finds no errors in my geometry. Please help.

Thanks.

Alex

SolidworksFlow Simulation